1. FIELD OF THE INVENTION
The present invention relates to quarts glass particularly excimer laser resistant quartz glass and a process for its production.
2. DISCUSSION OF BACKGROUND
Quartz glass is a material which is transparent in a wide wavelength regions from near infrared region to vacuum ultraviolet region, and has an extremely small thermal expansion coefficient, whereby it is excellent in dimensional stability. Furthermore, the quartz glass is excellent in chemical durability. Therefore, the quartz glass has been widely used as substrate material for photomask in a lithography step of production of LSI.
However, in conventional quartz glass, structural defects are induced when it is exposed to high energy of ultraviolet radiation such as plasma etching or excimer laser during a photomask processing step, whereby problems such as decrease of transmission in ultraviolet region, formation of fluorescence center, etc. are arised. Particularly, the conventional quartz glass has problems as substrate for photomask used in lithography of VLSI wherein ArF excimer laser or KrF excimer laser is used as light source of exposure. Further it has problems, as optical parts constituting an optical system wherein the above-mentioned excimer laser is used as light source.
Various attempts have been reported as means for solving the above-mentioned problems, and it has been known that such problems can be solved by having a hydrogen molecule contained in quartz glass in any arbitrary means. However, sufficient effects can not be necessarily expected unless quartz glass in which hydrogen is contained is specified. For example, Japanese Unexamined Patent Publication No. 201664/1989 discloses a method wherein quartz glass is subjected to heat treatment in an atmosphere containing hydrogen. However, it is impossible to completely suppress an absorption band in the vicinity of 260 nm and fluorescence in the vicinity of 650 nm formed when KrF laser is irradiated to quartz glass which is formed by transparent vitrification of a porous quartz glass product obtainable by flame hydrolysis of a glass-forming raw material.
On the other hand, as means for imparting to quartz glass excimer laser resistance, Japanese Unexamined Patent Publication No. 88742/1991 discloses a method wherein a hydrogen molecule is dissolved in quartz glass. However, the method is required to dissolve a large amount of hydrogen molecules in the quartz glass. Therefore, there is such a problem that the quartz glass is required to be subjected to heat treatment in a hydrogen atmosphere having risk of explosion, even under elevated pressure, whereby a large scale of equipment is necessary.
Further, a method for producing quartz glass which does not contain OH at all, wherein a porous quartz glass product obtained by flame hydrolysis of a glass-forming raw material is subjected to heat treatment in a halogen atmosphere, has been known, for example, as transparent vitrification method of porous quartz glass of VAD method, which is a method for producing low-loss quartz glass fibers. However, the quartz glass subjected to transparent vitrification has a strong absorption band in the vicinity of 250 nm and can not be used as substrate for photomask in lithography using KrF excimer laser. Furthermore, when the quartz glass is irradiated by KrF excimer laser, strong fluorescence is observed in the vicinity of 285 nm, 390 nm and 460 nm and the luminescence is blue by visual observation. In particular, if the quartz glass has fluorescence at 285 nm, it can not be used as substrate for photomask in lithography using KrF excimer laser as light source of exposure because the wavelength is within the photosensitive range of a resist.